The present invention relates to fluid cushion ground effect vehicles and more specifically to such vehicles which derive their lift by the dynamic action of an airfoil.
When a conventional airfoil is operated in a region close to the ground, its normal pressure distribution is distorted. Pressure tends to develop to a higher level under the wing and adds to the normal dynamic lift of the airfoil. This enhanced lift is well known as the ground effect.
There have been several versions of WIG vehicles developed over the years since the first one developed and patented by Finn Toivio Kaario in 1935. Some have had conventional high aspect ratio (AR.gtoreq.6) wings flying close to the surface, but their large wing span made them impractical machines to operate in geographically restrictive waterways such as harbors. Other versions incorporated low aspect ratio (AR.ltoreq.2) wing designs to keep the span within practical limits. Because small span wings have low aerodynamic efficiency, end plates were added to these low aspect ratio designs.
End plates, however, become hazardous in that they increase the danger of the wing "digging-in" as the end plates contact the water which becomes increasingly dangerous as speed operating ranges increase to those at which the WIG is most efficient (150 to 200 knots). So the WIG has a dilemma. If it flies high to avoid contacting waves, the "ground effect lift" diminishes rapidly and the WIG loses its advantage and becomes a poor airplane. If it flies low to maintain its aerodynamic advantage the end plates contact the water and high structural loads are imparted to the vehicle. A structure designed to withstand such loads must have increased weight and its consequent reduced payload capacity. In addition, the high structural loads seriously compromise the handling and maneuverability of the craft, thereby increasing the likelihood of pitch-in and capsizing.
Another difficulty experienced by WIG vehicles heretofore has been their inability to turn at high speed, a problem common to all high speed marine vehicles. Air cushion vehicles and surface effect ships have characterisitically large tactical diameters, hampering their military worth. The hydrofoil, in common with the airplane, enjoys smaller tactical diameters because of its ability to bank into a turn thereby producing large turning forces. The conventional WIG cannot bank into a turn because of its low surface clearance. Any attempt to bank at large angles to generate the turning force would cause the edge (tip) of the vehicle to dig in and overturn. This would especially be true of WIG vehicles with hard endplates.
Another disadvantage of most WIG designs is that since they gain their lift aerodynamically, they have no inherent hover capability. Such an added mode of operation designed into the vehicle would make it more useful. In some designs a pseudo hover capability is built in by the addition of floats for buoyancy lift (in water). These floats which also serve as the endplates cause the danger of water contact at high speeds referred to above.